The invention relates to making parts comprising a metal substrate provided with a protective coating forming a thermal barrier.
The field of application of the invention is making parts capable of retaining their mechanical properties at high temperatures, in particular gas turbine parts such as turbine blades, in particular for turbojet engines.
To improve the performance of gas turbines, and in particular the efficiency thereof, it is desirable to run them at temperatures that are as high as possible. To make parts for the hot portions, it is well known to use superalloys. As a general rule, superalloys comprise nickel as the main component and additional elements usually selected from chromium, cobalt, aluminum, molybdenum, titanium, tantalum, and many others.
Operating temperature can be further increased by providing the metal substrate of such parts with a protective coating that constitutes a thermal barrier.
It is known for this purpose to make a multilayer protective coating comprising an outer layer of ceramic and a metallic bonding underlayer, in particular an underlayer comprising aluminum or some other metal such as platinum.
The bonding underlayer interposed between the superalloy metal substrate and the ceramic outer layer serves to perform the following functions:                enabling an alumina film to be formed and to persist on its surface, which film has “adhesive” properties to enhance retention of the outer ceramic layer;        to protect the substrate from corrosion due to oxidation by the oxygen in any ambient medium that has managed to pass through the outer ceramic layer; and        to constitute a diffusion barrier against elements of the metal substrate which could contaminate the alumina film, thereby spoiling the interface between the bonding underlayer and the outer ceramic layer, and thus spoiling adhesion thereat.        
Including reactive elements such as yttrium, cerium, hafnium, or the lanthanides within the bonding underlayer reinforces the diffusion-barrier function and enhances persistence of the “adhesive” alumina film.
It is well known to form a bonding underlayer of the MCrAlY type (where M is a metal such as Fe, Ni, or Co) by using a physical vapor deposition method, e.g. by plasma sputtering, without giving rise to reaction with the substrate, adhesion between the bonding underlayer and the substrate being of a mechanical nature. By way of example, reference can be made to documents U.S. Pat. No. 4,055,705 and U.S. Pat. No. 5,824,423. Nevertheless, in order to obtain a thermally-stable underlayer, it is necessary for it to be relatively thick, typically not less than 50 micrometers (μm) to 100 μm, and that gives rise to a weight penalty.
Other known methods consist in making the bonding underlayer out of an intermetallic compound which can be of smaller thickness due to its thermal stability. An intermetallic compound comprising aluminum and platinum has been found to have good properties.
Thus, U.S. Pat. No. 5,716,720 describes a method consisting in forming a platinum layer electrolytically on a nickel-based superalloy substrate, and subsequently in performing vapor aluminization at a temperature higher than 1000 degrees Celsius (C). Nickel from the substrate diffuses within the bonding underlayer. An alumina film is formed by heat treatment on the surface of the bonding underlayer prior to forming a ceramic outer layer, e.g. out of ytrried zirconia obtained by physical vapor deposition. A reactive element can be included in the bonding underlayer during the step of vapor aluminization.
U.S. Pat. No. 5,238,752 describes another method which consists in forming a bonding underlayer on a superalloy substrate in which the underlayer comprises an intermetallic compound, in particular a compound of aluminum and platinum. The bonding underlayer is made by pack cementation at a temperature higher than 985° C. and it has a thickness greater than 25 μm. An alumina film is formed by oxidation on the surface of the bonding underlayer prior to forming the ceramic outer layer, e.g. of yttried zirconia by physical vapor deposition.
Patent application EP 0 985 744 describes yet another method comprising forming a layer of platinum on a nickel-based superalloy substrate by electrodeposition or by chemical vapor deposition and depositing an aluminum layer which is made from a gaseous halide and which diffuses into the platinum layer. Desulfurization and surface descaling is performed after each deposition operation by heat treatment at a temperature higher than 1050° C. in order to eliminate sulfur which is harmful to adhesion of the alumina film that develops on the surface of the resulting bonding underlayer. At that temperature higher than 1050° C., it is inevitable that elements in the substrate will diffuse into the bonding underlayer.
A method of forming a bonding underlayer comprising platinum and aluminum is also described in U.S. Pat. No. 5,856,027. A platinum layer is formed on the superalloy substrate by electrodeposition prior to depositing aluminum by chemical vapor deposition, optionally together with reactive elements. The resulting underlayer presents an internal diffusion zone in which nickel diffused from the substrate is present.
With those known methods, the way in which the bonding substrate is formed gives rise to interaction with the substrate. The Applicant has observed that various elements of the superalloy substrate diffusing into the bonding underlayer can form undesirable precipitates therein that are liable in particular to affect the alumina film developed on the surface of the bonding underlayer. In addition, it is difficult to control the precise composition of the bonding underlayer.